HiveMail 1.3 actually includes a spelling feature, althought it "independent" in that it doesn't use any external libraries to perform the spell checking. It's actually pretty damn fast, but if you have access to install pspell or aspell on this server you should probably use those. As for the interface, you can easily rip HiveMail's and just change the underlying code to make calls to the libraries instead of checking the words through PHP.